The profile of spontaneous, endogenously-engendered disease is being tabulated in germfree rats, and compared to the disease spectrum in conventional counterpart rats. The predominant lesions observed in aged germfree Wistar rats involved endocrine glands and their target organs. In rats over 30 months of age, 80% had adenomas of the pituitary and adrenal glands, with reducing frequencies in thymus, thyroid, parathyroid, and liver glands. Three malignant prostate adenocarcinomas have been observed in them. Few conventional rats live beyond 24 months of age, but endocrine lesions have been observed among them. The exposures of germfree rats to environmental pollutants continues.